1. Field of the Invention
The present invention relates to a serial head type recorder using a serial head.
2. Description of the Prior Art
FIG. 34 is a perspective view showing a principal part of a serial head type recorder, and FIG. 35 is an illustration showing a printing mechanism. A paper conveying section 501 comprises a conveying roller 501a and a pinch roller 501b. The pinch roller 501b and the conveying roller 501a are provided in an opposite arrangement, to convey a paper sheet 510 with the paper sheet 510 interposed therebetween. A platen 502 is arranged in the vicinity of the paper conveying section 501. A paper feeding roller 508 feeds the leading end of the paper sheet 510 to the paper conveying section 501. A pulse motor 511 (not shown in FIG. 34. See FIG. 36) is rotated through a predetermined angle in response to the number of pulses. A group of gears 503 transmits torque produced by the pulse motor 511 to the conveying roller 501a and the paper feeding roller 508. A serial head 504 does printing on the paper sheet 510, which is of a thermal type, for example. A head supporting member 515 for supporting the serial head 504 is mounted on a carriage 505. An ink ribbon cassette 506 is set in the carriage 505. A shaft 507 guides the carriage 505 in transverse direction (cross the direction of paper carriage at right angle) of the paper sheet. The paper sheet is put on a paper feeding plate 512.
The paper sheet 510 is positioned between the serial head 504 and the platen 502. An ink ribbon 506a of the ink ribbon cassette 506 is interposed between the paper sheet 510 and the serial head 504. The serial head 504 and the ink ribbon cassette 506 are moved leftward (in the direction of carriage scanning) in FIG. 35. The ink ribbon 506a is wound around a winding-side roll 506c upon being delivered from a feeding-side roll 506b. After one line is printed, the conveying roller 501a is driven, so that the paper sheet 510 is moved by its line feed width (that is, a line feed operation is performed).
FIG. 37 is a schematic view showing how printing of three lines and approximately one-third of the subsequent line is done on the paper sheet 510 by the above-mentioned printing principle.
A color printer can be also constructed using the above-mentioned serial head by preparing ink ribbons in colors such as yellow (Y), magenta (M), and cyan (C) or preparing an ink ribbon on which colors such as yellow (Y), magenta (M) and cyan (C) are printed in this order. Formation of a color image in such a color printer can be realized by first printing one scanning and recording line by the ink ribbon in yellow (Y), then printing a scanning and recording line over the above-mentioned scanning and recording line by the ink ribbon in magenta (M), and further printing a scanning and recording line over the above-mentioned scanning and recording line by the ink ribbon in cyan (C), to form one color scanning and recording line, and repeating the printing of the color scanning and recording line.
FIG. 36 is a perspective view showing a conventional paper conveying mechanism. A driving force produced by the pulse motor 511 is respectively transmitted to the conveying roller 501a and the paper feeding roller 508 through a first transmission gear system 531 and a second transmission gear system 532 in the group of gears 503. A paper feeding gear 532a in the second transmission gear system 532 and the paper feeding roller 508 are directly connected to each other.
In the serial head type recorder, non-uniformity in a pitch between lines printed presents a problem. For example, if the amount of conveyance of the paper sheet 510 is larger than the line feed width (the printing width of the serial head 504), a clearance occurs between the lines, as shown in FIG. 38. On the other hand, if it is smaller, images are undesirably overlapped with each other, so that the density of only a portion where the images are overlapped with each other is increased, and the images are degraded because dots do not coincide with each other. FIG. 38 is a schematic view. In a sublimation type printer, an ink layer as shown in FIG. 38 is not seen.
The non-uniformity in a pitch between lines is caused by the non-uniformity in conveyance of a paper sheet in a paper conveying system. The causes of the non-uniformity in conveyance are (1) eccentricity of the conveying roller 501a and gears, and (2) the load of a paper sheet from the exterior.
The above-mentioned item (1) will be considered. FIG. 39 is an illustration showing the relationship between non-uniformity in conveyance and deviation between lines. S denotes the target amount of conveyance, and L denotes the actual amount of conveyance. The difference between S and L is non-uniformity in conveyance (F=L-S). Letting G be a line feed width, a clearance between the 0-th line and the first line is L1-(L0+G). L0=S0+F0, L1=S1+F1, and G=S1-S0, whereby the clearance is represented by F1-F0. FIG. 40 is a graph showing the relationship between the number of pulses fed to a motor and the amount of paper feeding, where a solid line indicates the ideal amount of feeding (no non-uniformity in conveyance), and a dotted line indicates the actual amount of feeding (non-uniformity in conveyance).
In order to solve the problem described in the above-mentioned item (1), an attempt to employ a high-precision gear and a high-precision roller has been made. Even in such a case, however, non-uniformity in conveyance of approximately 65.+-..mu.m occurs. In a high-resolution printer and a color printer, therefore, only the employment of such a method is insufficient as a countermeasure of the non-uniformity in a pitch between lines. A method of intentionally overlapping dots on the preceding line in the case of a line feed operation to prevent the occurrence of a clearance between lines has been known. Such a method is effective to a certain extent in a melt type color printer. In the sublimation type color printer, however, when dots are thus overlapped with each other, an image of good quality is not obtained unless the amount of deviation in overlapping of dots is changed from zero to not more than approximately 30 .mu.m in the direction of overlapping.
The load described in the item (2) will be then considered. Examples of the load applied to the paper sheet 510 include a load caused by the paper feeding roller 508, a frictional force produced between paper sheets stocked in the paper feeding plate 512 and a paper sheet which is being printed, and a frictional force with a guide portion on a paper path. The largest one of them is the load caused by the paper feeding roller 508. The load caused by the paper feeding roller 508 is not produced if the peripheral speed thereof (Vf: see FIG. 36) is made entirely equal to the peripheral speed of the conveying roller 501a (Vc: see FIG. 36), which is actually difficult. There is slack in conveyance in the paper sheet 510 in the case of Vf&gt;Vc, while the above-mentioned load, that is, a force for the conveying roller 508 to return the paper sheet 510 is produced in the case of Vf&lt;Vc.
Conventionally, a contact state between the paper sheet 510 and the paper feeding roller 508 has been maintained until the leading end of the paper sheet 510 is detected by a sensor (not shown) through the paper feeding section 501, as shown in FIG. 41A, while the contact state between the paper sheet 510 and the paper feeding roller 508 has been released upon swinging the paper feeding plate 512 downward, as shown in FIG. 41B. An example of a mechanism for swinging and driving the paper feeding plate 512 is a mechanism comprising a spring, a push-down cam, and a electromagnetic clutch for controlling the push-down cam.
FIG. 42 is a flow chart showing the contents of control in a recorder having the above-mentioned swing-type paper feeding plate. The paper feeding plate 512 is first swung upward, to form the contact state between the paper sheet 510 and the paper feeding roller 508 (step 521). The pulse motor 511 is then driven, to rotate the paper feeding roller 508 and the conveying roller 501a (step 522). It is judged whether or not the leading end of the paper sheet 510 is detected (step 523). If the leading end of the paper sheet 510 is detected, the driving of the pulse motor 511 is stopped (step 524), and the paper feeding plate 512 is swung downward to release the contact state (step 525). One line is printed in three colors (step 526), it is judged whether or not printing of all lines is terminated (step 527), a paper discharging operation is performed if the printing of all lines is terminated (step 528), and a line feed operation is performed if the printing of all lines is not terminated (step 529), after which the program proceeds to the step 526.
The other prior art has been also known. FIG. 43 is a perspective view showing the other conventional paper conveying mechanism. A driving force produced by a pulse motor 511 is respectively transmitted to a conveying roller 501a and a paper feeding roller 508 through a first transmission gear system 531 and a second transmission gear system 532 in a group of gears 503. A paper feeding gear 532a in the second transmission gear system 532 and the paper feeding roller 508 are connected to each other through a spring clutch 513. A spring cylindrical portion of the spring clutch 513 is fitted in a shaft portion 508a of the paper feeding roller 508, and one end of the spring clutch 513 is fastened to the paper feeding gear 532a. In a state where the rotational speed of the paper feeding gear 532a exceeds the rotational speed of the paper feeding roller 508, the spring clutch 513 is strongly fastened to the shaft portion 508a, to feed the driving force to the paper feeding roller 508. On the other hand, in a state where the rotational speed of the paper feeding gear 532a is below the rotational speed of the paper feeding roller 508, a fastening force to the shaft portion 508a is weakened, to allow the free rotation of the paper feeding roller 508.
The ratio of gears in the group of gears 503, for example, is so set that the peripheral speed Vf of the paper feeding roller 508&lt;the peripheral speed Vc of the conveying roller 501a.
FIG. 44 is a flow chart showing the contents of control at the time of printing in a recorder comprising the above-mentioned paper conveying mechanism. Part of operations (states) which are not control operations are described, and are denoted by S'. The driving of the pulse motor 511 is first started, to rotate the paper feeding roller 508 and the conveying roller 501a (step 531). The speed relationship therebetween is the peripheral speed Vf of the paper feeding roller 508&lt;the peripheral speed Vc of the conveying roller 501a (S'532), as described above. This state is maintained until the leading end of the paper sheet 510 is interposed between the conveying roller 501a and the pinch roller 501b (S'532, S'533).
When the leading end of the paper sheet 510 is interposed between the conveying roller 501a and the pinch roller 501b, the paper sheet 510 is conveyed at the peripheral speed Vc of the conveying roller 501a and the paper feeding roller 508 is rotated at the peripheral speed Vc upon being dragged by the friction of the paper sheet 510. Consequently, the rotational speed of the paper feeding gear 532a is below the rotational speed of the paper feeding roller 508, so that a fastening force to the shaft portion 508a is weakened. Therefore, the paper feeding roller 508 is rotated by the movement of the paper sheet 510 upon permission of the free rotation of the paper feeding roller 508, so that paper feeding roller 508 and the conveying roller 501a are rotated at an equal peripheral speed (S'533, S'534).
It is then judged whether or not the leading end of the paper sheet 510 is detected (step 535). If the leading end of the paper sheet 510 is detected, the pulse motor 511 is stopped (step 536). One line is printed in three colors (step 537), and it is judged whether or not printing of all lines is terminated (step 538). If the printing of all lines is terminated, a paper discharging operation is performed (step 539). On the other hand, if the printing of all lines is not terminated, a line feed operation is performed (step 540). When the trailing end of the paper sheet 510 deviates from the paper feeding roller 508, the paper sheet 510 is conveyed only by the conveying roller 501a (S'541, S'542). Before the trailing end of the paper sheet 510 deviates from the paper feeding roller 508, a state where the paper feeding roller 508 and the conveying roller 501a are rotated at an equal peripheral speed is maintained (S'541, S'543).
In the above-mentioned conventional structure in which the paper feeding plate 512 is swung, the load at the time of printing (at the time of a line feed operation) can be almost eliminated. However, a swinging and driving mechanism is required, making the structure complicated, so that the cost is comparatively high. Further, time required for a swinging operation increases time required for printing.
In the conventional structure in which the spring clutch 513 is used, the paper feeding roller 508 is rotated by the movement of the paper sheet 510 upon permission of the free rotation of the paper feeding roller 508. Therefore, the load applied to the conveying roller 501a is reduced. However, such a coupled driven operation itself will be a certain degree of load. Moreover, sliding friction between the shaft portion 508a and the spring clutch 513 increases the load in the case of the coupled driven operation, whereby a slip, that is, non-uniformity in conveyance is liable to occur at the time of a line feed operation of the paper sheet 510 by the conveying roller 501a. In experiments, a slip of several hundred micrometers in width occurs with respect to a line feed width of 17 mm, so that the image quality cannot be sufficiently improved.
The present invention has been made in view of the above-mentioned circumstances and has for its object to provide a serial head type recorder capable of reducing non-uniformity in conveyance to improve the image quality.